The present invention relates generally to the field of fluid drop generation and more specifically to jet drop recorders of the type shown generally in U.S. Pat. No. 3,710,998 to Mathis, issued Oct. 31, 1972 and assigned to the assignee of the present invention. In recorders of this type, a number of orifices are provided to receive an electrically conductive recording fluid, such as a water base ink, from a pressurized fluid manifold and to eject the fluid in parallel streams. The fluid flows through the orifices in a plate with the formation of drops being stimulated by the application to the plate of a series of transverse waves. This method of drop generation is more completely described in U.S. Pat. No. 3,739,393 to Lyon et al., issued June 12, 1973, and assigned to the assignee of the present invention.
Graphic reproduction in recorders of this type is accomplished by depositing selected ones of the drops in the drop streams on a moving web of paper or other material. The drops are selectively charged prior to subjecting the stream to a drop-deflecting electric field. The charged drops are deflected by the field while the uncharged drops pass through the field unaffected. Printing is accomplished by allowing either the group of charged drops or the group of uncharged drops to strike the web. A catcher is positioned in the trajectory of the other group of drops such that they are caught and thereby prevented from striking the web.
Charging of the drops is accomplished by the application of discrete charge control signals to charging electrodes near the break off point of the drop streams. As the drops separate from their parent fluid filaments, they carry a portion of the charge induced in the filament by the associated charging electrode.
The charging electrodes have typically comprised orifices in a non-conductive charge ring plate which are lined with a conductive material, such as gold, to form charge rings. Such a prior art charge plate arrangement is shown and described more completely in U.S. Pat. No. 3,701,998 issued Oct. 31, 1972, to Mathis and assigned to the assignee of the present invention. Other charging electrodes have been configured as notched plates such as shown in U.S. Pat. No. 3,618,858, issued Nov. 9, 1971, to Culp and assigned to the assignee of the present invention.
Whatever the exact configuration utilized, however, prior art charge electrodes have been subject to failure as a result of deplating currents between adjacent electrodes. The apparatus generating the fluid drops may also generate droplets of small size which form an ink mist. While very little of this mist will be present at any one time, it is possible that an amount of the conductive ink sufficient to carry a current may be deposited on the surface between adjacent electrodes. When a potential exists between the electrodes, a current will flow and, as a result, the charge electrodes may be totally or partially deplated. A partially deplating electrode will be less effective in inducing a charge in the ink drops passing the electrode and will result in degradation of the print image.
One approach which as been taken to eliminate this deplating process is shown in U.S. Pat. No. 3,604,980, issued Sept. 14, 1971, to Robertson, and assigned to the assignee of the present invention. Robertson suggests the use of an insulating coating over the surface of the charging electrode to prevent adjacent electrodes from shorting out. Such a configuration may, however, be disadvantageous in that a charge may build up inside the ring and may not be readily dissipated. This will, of course, inhibit rapid switching of the charge electrodes.